Various fuel vapor control apparatus are heretofore well known. One example of such apparatus which is disclosed in, for example, Japanese Utility Model Publication No. 61-23,644 entitled "Fuel Vapor Treatment Apparatus" published on July 15, 1986, includes a vapor flow control valve which delivers variably in amount fuel vapor from a vapor storage canister collecting fuel vapor produced in a fuel tank by an absorbtion process to the air intake chamber of an automotive vehicle engine. This vapor flow control valve acts to increasingly or decreasingly change the amount of fuel vapor passing therethrough and delivered to an intake airstream depending upon the amount of intake air flowing an intake air pipe. The provision of such a fuel vapor control apparatus enables the automotive engine to consume promptly the whole of fuel vapor in the vapor storage canister for the combustion of air-fuel mixture conducted in an engine combustion chamber. A decreased amount of fuel vapor delivered into the air intake chamber can highly improve the combustibility of air-fuel mixture upon delivery of a small amount of intake air into the air intake chamber which is caused while the automotive vehicle engine idles. On the other hand, the vapor storage canister can be always be ready for sufficiently collecting and storing even a large amount of fuel vapor from the fuel tank by delivering an increased amount of fuel vapor into the air intake chamber while engine load is high and a large amount of intake air is conducted, thereby preventing the vapor storage canister from being filled with fuel vapor to exceed its capacity and overflow and fuel vapor from emitting into the atmosphere.
Meanwhile, as is well known to those having an ordinary skill in the art, with an increase of the amount of intake air caused due to an increasing change of throttle valve opening during an acceleration of the automotive vehicle engine, a negative pressure of intake air flowing directly behind the throttle valve will not rapidly, but rather gradually, decrease or rise close to the atmospheric pressure and, in strict perspective, intake air flowing behind the throttle valve characteristically shows a retardation of the rise of pressure or a rise of pressure behind an increasing change of the amount of intake air, accordingly.
The above described fuel vapor control apparatus utilizing vapor control valves used hitherto has, on one hand, a significantly advantageous function to increasingly vary the available vapor flow depending upon an increase of the amount of intake air conducted in the intake air chamber while the engine is under acceleration but is, on the other hand, associated with a problem that an excessive amount of fuel vapor is delivered to air-fuel mixture at the beginning of engine acceleration as a result of a retarded rise in pressure of intake air. This excessive delivery of fuel vapor will certainly impede a desired, ideal combustion of air-fuel mixture, resulting in a low operation efficiency of the engine.